1. Field of the Invention
This invention is directed generally to an apparatus for growing large single crystals of metal halides and chalcogenides, in a controlled environment in general and more particularly to the fabrication of a crystal growth apparatus for use in the growth of large single crystals in a controlled environment of a specific character.
2. Prior Art
Crystal growth crucibles employed in prior art crystal growth from the melt processes may be characterized as opened or closed end. These devices are especially fabricated by individual researchers from vitreous silica, or graphite materials which are readily available in most laboratories. There are no commercial suppliers of crystal growth crucibles known to Applicant. Examples of prior art crucibles employed in prior art crystal growth processes are those employed by Bridgman, Stockbarger, Czochralski and their various modifications by Dillon, Davey, Quimby, Kapitza, Gompery and Nacken (see "Crystal Growth" by H. E. Buckley, pp. 71 et seq, published by John Wiley & Sons, Inc.).
Prior art crystal growth crucibles were not suitable for crystal growth under a reactive atmosphere or other controlled atmospheric condition because of the propensity of these cruicibles for reacting with the corrosive gases of the reactive atmosphere.
The necessity of controlling the growth environment may be understood when one takes into consideration the effect of trace impurities and/or contaminants on the ability to produce large optically pure single crystals and the stability of such crystals when employed in high power laser applications.
In the well known and popular Bridgman-Stockbarger method, the melt and the heating element "see" the same atmosphere. Therefore, both the melt and the heating element must be chemically compatible with their common atmosphere, so that this atmosphere will not serve as a chemical bridge for mass exchange between the melt and the heater. Otherwise, problems of contamination and/or heater stability will set in.
Chemical compatibility between the melt, the container structural materials and the heating elements of the furnace is a severe constraint in the production of large ultra pure single crystals. Since the spectrum of choices of heating elements available in the market is narrow, the freedom of choice of crystal material is limited. The need for precise temperature control during the growth process has long been recognized by those skilled in the art. For example, the Bridgman method, as improved upon by Stockbarger, utilizes dual furnaces separated by a platinum baffle to create sharp thermal gradients as the crucible is lowered during the crystal grown process. However, it can be shown that the sharp thermal gradient, obtained by this process, exist only at the interior surface of the furnace. The temperature profile along the axis of the charge material is quite different, unless the charge material is an excellent thermal conductor, whether it is frozen or melted.
Heat losses due to radiation continue to predominate in Bridgman-Stockbarger growth methods. Concave upward isotherms are formed as a result of the flow of heat towards the sides of the crucible. Since crystal growth takes place at the interface of the isotherm at the melting point (T.sub.mp), the crystal will grow along a concave upward surface. This phenomena tends to yield less perfect crystals because of stress inducement.